IL-2 and IL-15 function in aiding the proliferation and differentiation of B cells, T cells, and NK cells. IL-2 is also essential for regulatory T cell (Treg) function and survival. Both cytokines exert their cell signaling function through binding to a trimeric complex consisting of two shared receptors, the common gamma chain (γc; CD132) and IL-2 receptor B-chain (IL-2Rβ; CD122), as well as an alpha chain receptor unique to each cytokine: IL-2 receptor alpha (IL-2Rα; CD25) or IL-15 receptor alpha (IL-15Rα; CD215). Both cytokines are considered as potentially valuable therapeutics in oncology and IL-2 has been approved for use in patients with metastatic renal-cell carcinoma and malignant melanoma. Currently there are no approved uses of recombinant IL-15, although several clinical trials are ongoing.
IL-2 presents several challenges as a therapeutic agent. First, it preferentially activates T cells that express the high affinity receptor complex, which depends on CD25 expression. Because Treg cells constitutively express CD25, they compete for IL-2 supplies with effector T cells, whose activation is preferred for oncology treatment. This imbalance has led to the concept of high dose IL-2. However, this approach creates additional problems because of IL-2-mediated toxicities such as vascular leak syndrome.
IL-2 is secreted primarily by activated T cells, while its receptors are located on activated T cells, Tregs, NK cells, and B cells. In contrast, IL-15 is produced on monocytes and dendritic cells and is primarily presented as a membrane-bound heterodimeric complex with IL-15Rα present on the same cells. Its effects are realized through trans-presentation of the IL-15/IL-15Rα complex to NK cells and CD8+ T cells expressing IL-2Rβ and the common gamma chain.
As potential drugs, both cytokines suffer from a very fast clearance, with half-lives measured in minutes. In addition, IL-15 by itself is less stable due to its preference for the IL-15Rα-associated complex. It has also been shown that recombinantly produced IL15/IL15Rα heterodimer can potently activate T cells. Nevertheless, a short half-life hinders favorable dosing. The present invention solves this problem by providing novel IL15/IL15Rα heterodimer Fc fusion proteins.